The invention relates broadly to fault detection systems, and more particularly to a system that is utilized in connection with an electrically heated vacuum furnace to detect faults such as carbon buildup, a load touching a heating element, short circuits from broken heating elements, and short circuits in any heating element connection, including single phase, three phase delta, and three phase wye connections. Applicants are not aware of any system other than theirs, for sensing all of the above-described faults which may occur during operation of a vacuum furnace using any of the aforementioned connections.
Briefly stated, the invention is in a device for detecting in an electrically heated furnace any of the aforementioned faults. The device essentially comprises, (I) an electrically balanced bridge having one of its resistant legs in electrical communication with the electric heating element and the grounded casing of the furnace, (II) means for periodically shutting off the flow of electric current to the heating elements, and (III) means for detecting leakage of electrical current from the bridge when any of the aforementioned faults causes an unbalancing of the bridge.
Means are provided for continuously monitoring the circuit, including the electrical heating element and casing, for the flow of alternating current to the bridge and for maintaining the circuit open to prevent destruction of the bridge when there is an alternating current in the circuit. Because of the weakness of the electrical current leaking from the bridge, means are provided for amplifying the leakage for detection and subsequent use in a visual readout device or, in more sophisticated systems, to shut down operation of the furnace by stopping the flow of alternating current to the electrical heating element.